The invention relates to a nonlinear optical compound which comprises an electron donor group and an electron acceptor group which are linked by a .pi.-conjugated system.
The invention also relates to a nonlinear optical medium which comprises a polymer matrix and polarly ordered nonlinear optical compounds, said nonlinear optical compounds comprising an electron donor group and an electron acceptor group which are linked by a .pi.-conjugated system.
The invention further relates to a method of manufacturing a nonlinear optical medium.
The invention also relates to a device for doubling the frequency of a light wave, a fundamental light wave being led through a nonlinear optical medium, so as to form a second harmonic wave.
Such a compound and a device in the form of a planar waveguide are described in European Patent Application EP 254921, in which a fundamental light wave is generated by means of an YAG laser having a wavelength of 1064 nm. Crystalline methylnitroaniline (MNA) is used as the nonlinear optical compound. By means of the device, green light having a wavelength of 532 nm is obtained. The light wave thus obtained, whose frequency is doubled, is called second harmonic wave.
Another known nonlinear optical compound is 4-dimethylamino-4'-nitrostilbene (abbreviation: DANS). Compounds such as MNA and DANS are called D.pi.A compounds, wherein D denotes an electron donor group such as an amino group, A denotes an electron acceptor group such as a nitro group and .pi. denotes a .pi.-conjugated system such as a benzene ring. Nonlinear optical (NLO) effects which can be generated with such compounds are the electrooptical effect (Pockels effect) on which electrooptical switches are based and frequency-doubling of laser light. At molecular level, the NLO effect is characterized by the hyperpolarisability .beta.; at a macroscopic level (many molecules) the NLO effect is characterized by second order nonlinear optical susceptibility .chi..sup.(2). Non-NLO compounds have a value .beta.=0. Only compounds having a high value of .beta. are important in NLO applications. At macroscopic level the same applies to .chi..sup.(2). The above-mentioned D.pi.A compounds have a relatively high .beta. value.
A disadvantage of the above-mentioned and many other NLO compounds is that they are unsuitable for doubling the frequency of laser light having a wavelength of approximately 800 nm, because these compounds exhibit a considerable absorption for wavelengths having a double frequency, i.e., a frequency of approximately 400 nm. As a result, the output of blue laser light in nihil. In practice there is a substantially need for blue laser light, for example, in the field of optical recording. Blue light can be focused more sharply than red light because the size of the focus decreases with shorter wavelengths. In the case of optical recording, the information density increases by a factor of four when blue light is used instead of red light. Also in the case of photochemical processes, blue laser light is more interesting than red laser light because of the larger photochemical activity of blue light. The above-mentioned compounds can, however, be used to generate second harmonic light waves having a wavelength above approximately 530 nm (green), and in applications for electrooptical switches for wavelengths of, for example, 800 nm, 1.06 .mu.m, and 1.3 .mu.m.